The present invention relates generally to a method for measuring impedance and other parameters of electrical distribution equipment at harmonic frequencies.
Line impedance forms the basis for many power quality calculations, including voltage sag due to fault and inrush currents, and steady-state line voltage distortion caused by load current distortion. While these calculations have traditionally been performed using the fundamental frequency value of line impedance, it has been recognized as desirable to measure impedance at non-fundamental, or harmonic, frequencies. It is generally advantageous to measure impedance while the power distribution system is energized, as this avoids expensive down time. However, impedance calculations are considerably more difficult in an energized system.
The article by A. W. Kelley et al., xe2x80x9cComplete Characterization of Utilization-Voltage Power System Impedance Using Wideband Measurementxe2x80x9d, 1996 IEEE Industrial and Commercial Power Systems Conference, and U.S. Pat. No. 5,587,662 to Kelley et al. discloses an apparatus which includes a signal source and a power amplifier. The apparatus injects voltage at various frequencies into the power system, and measures the resulting currents. An associated computer plots graphs of resistance and inductance versus frequency. Readings near 60 Hz (and at 180 Hz, and other harmonic frequencies) are inaccurate because the line voltage interferes with measuring the component of the voltage resulting from the injected current. The Kelley article suggests that the values near 60-Hz (and other harmonic frequencies) can be interpolated from the values below 40 Hz and above 80 Hz.
This approach has limitations in accuracy and flexibility. For example, there is often a xe2x80x9cresonance pointxe2x80x9d which will limit the range of values that can be used for interpolation. In other words, the range of frequencies used for interpolation has an upper limit that must be below where resonance effects due to normal system inductance and capacitance become significant.
It would be desirable for a meter to be capable of scanning harmonic frequencies to be used for measurements and to automatically select appropriate frequencies where no interference exists. It would further be desirable for a meter to measure line voltage and to use the line voltage measurement to calculate available Mega Volt-Amperes (MVA). MVA is typically calculated as the product of System Line-Line Voltage in kiloVolts (kV) and Available Symmetrical RMS Fault Current in kiloAmperes (kA)xc3x973. It would also be desirable for a meter to be usable in three-phase systems.
The present invention overcomes the above-noted disadvantages of prior devices and methods, and achieves additional advantages, by providing a method for measuring the impedance of electrical distribution equipment. The method comprises applying a voltage at a first selected frequency to electrical distribution equipment, waiting a delay time for transient effects to settle, measuring current through the electrical distribution equipment, and selecting additional frequencies to repeat the steps of applying, waiting, and measuring. The method can be implemented, for example, by a meter which includes: a control processor connected to receive and execute instructions, and to output a control signal; a frequency generator for generating a frequency signal having a frequency determined by the control signal; a power amplifier connected to receive the control signal and the frequency signal, the power amplifier outputting a voltage signal to the electrical distribution system through a voltage transformer; and an analog-to-digital converter connected to receive analog feedback voltage signals from the voltage transformer, the analog-to-digital converter outputting a digital feedback voltage signal to the processor.
The present invention advantageously provides, in a single integrated device, the ability to monitor electrical distribution equipment accurately and reliably.